boost, shared ptr Vs weak ptr? Which to use when?

In my current project I am using boost::shared_ptr quite extensively.

Recently my fellow team mates have also started using weak_ptr. I don't know which one to use and when.

Apart from this, what should I do if I want to convert weak_ptr to shared_ptr. Does putting a lock on weak_ptr to create a shared_ptr affect my code in other thread?